System and Method for Precise, Accurate and Stable Optical Timing Information Definition Including Internally Self-consistent Substantially Jitter Free Timing Reference

ABSTRACT

An optoelectronic timing system includes an optical timing compensation system in which optical pulses from a semiconductor laser are advanced or retarded based upon an expected arrival time. The pulses are directed into a number of time-quantifiable optical paths. Optical switches may direct a pulse into an advancing path or a retarding path based on an arrival time of a previous pulse. The optical compensation system may be incorporated into a precision timing device in which multiple optical paths are arranged so that a travel time of a path is one order of magnitude different than a travel time of an adjacent path. Timing signals can be developed by coupling an optical detector to each of the multiple optical paths.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 10/692,175, filed Dec. 28, 2002, now U.S. Pat. No. 8,358,931, whichis related to and takes priority from U.S. Provisional PatentApplication, Ser. No. 60/434,539, filed Dec. 18, 2002, entitled “SystemAnd Method For Precise, Accurate And Stable Optical Timing InformationDefinition” and is further related to U.S. patent application Ser. No.10/691,869, entitled “System And Method For Developing High Output PowerNanosecond Range Pulses from Continuous Wave Semiconductor LaserSystems”, now U.S. Pat. No. 7,869,477, and U.S. patent application Ser.No. 10/692,176, filed Oct. 23, 2003, entitled “System and Method forPrecise, Accurate and Stable Optical Timing Information Definition”, nowU.S. Pat. No. 8,068,743, all commonly owned by the assignee of thepresent invention, the entire contents of which are expresslyincorporated herein by reference.

FIELD OF THE INVENTION

The present invention is directed generally to optical timing systemsand, more particularly to systems and methods for generation andprocessing of high speed native timing signals in the gigahertz region.

BACKGROUND OF THE INVENTION

Internationally, telecommunications is undergoing major rapid changebrought about by regulatory changes, increasingly open markets andtechnological advances in integrated circuits, optical devices, andcomputer systems. The convergence and the integration of thesetechnologies, coupled with the driving factors of faster transmissionspeeds, lower signal levels, and denser circuit boards has made managingsignals in electronic and communication switching systems critical.

These driving factors have placed a greater emphasis on managingproblems relating to signal integrity, timing distribution, timingjitter, signal distribution, noise, asynchronism, and cross-talk. Inlong-haul transmission, domain optical amplifiers together withwavelength-division multiplexing have revolutionized high speed datatransmission by providing flexible and cost-effective means ofamplifying and processing of signals almost entirely in the opticaldomain independent of data rate and protocols.

Impairments suffered by timing signals play a critical role inelectronic systems. They limit the dynamic range of ananalogue-to-digital converter, the throughput of a digital bus, affectthe behavior of digital synchronizers, influence the bit error ratio ofa communications link, and determine the sensitivity and selectivity ofradio receivers. Timing impairments are the result of random noise andsystematic disturbances within electronic devices and interconnections.

Electronically derived timing signals suffer from an additionalinherency constraint; the limits of electronic system frequency responseis predicated on the internal parasitic capacitances developed as anartifact of the functional underpinnings of active semiconductorintegrated circuit devices. The familiar P-N junction which forms thebasis of active device fabrication, whether expressed in terms ofmajority or minority carrier devices, nevertheless inherently compels aparasitic capacitance be coupled into the elemental circuit model.

Thus, in the rapidly advancing telecommunications field, for example,electronically generated timing signals are becoming increasinglyproblematic as fundamental limits of integrated circuit frequencyresponse are reached. However, the burgeoning field of optoelectronicsoffers a means of avoiding a strict dependence on electrical/electronictiming signal generation. Optoelectronics is predicated upon opticalsignal processing and inherently includes technologically satisfactorystructures for confining and transmitting optical pulses over greatdistances. Since the speed of light has a well recognized constantvalue, given a particular transmission medium, a light pulse can beutilized to define a non-relative and non-relativistic methodology formeasuring time as well as time intervals. A light pulse traveling at aconstant velocity, traversing a known distance, in the same referenceframe as an observer, provides a simple and inherently stable method fordefining a time interval. Mechanical definition of a multiplicity ofbranching travel paths offers a straight forward way of constructing atiming generator characterized by timing trigger edges having nativeperiodicities in the gigahertz and multi-gigahertz regime.

SUMMARY OF THE INVENTION

In an optoelectronic timing system, an adaptive frequency generatorsystem includes at least one semiconductor laser configured to issuesubnanosecond optical pulses defining a periodic pulse train. At least afirst optical waveguide is configured to define a firsttime-quantifiable optical path for a pulse of the train and at least oneadditional optical waveguide is configured to define a secondtime-quantifiable optical path for a pulse of the train different fromthe first waveguide.

Pulses of the train are directed into the first and second waveguides ata first nodal point coupled to the first and second waveguides andpulses directed into the first and second waveguides are recombined at asecond nodal point coupled to the first and second waveguides. Thelength of the second time-quantifiable optical path has a definednumerical relationship to the length of the first time-quantifiableoptical path, such that the periodicity of pulses recombined at thesecond nodal point has the same numerical relationship with theperiodicity of the issued pulse train.

In one aspect of the invention, the at least one semiconductor laser isconfigured to provide a pulsed output having a periodicity in the rangeof about 1 nanosecond so as to define a 1 gigahertz pulse train.Additionally, the second optical time-quantifiable optical path has alength differing from the first time-quantifiable optical path by about0.5 nanoseconds, so as to define a 2 gigahertz pulse train at the secondnodal point.

One feature of the present invention allows expansion of the number oftime-quantifiable optical paths to provide for adaptively shorterperiodicities. In this aspect, one system includes multiple additionaloptical waveguides each coupled to the first and second nodal points,the additional waveguides configured to define multipletime-quantifiable optical paths. The lengths of each of the additionaltime-quantifiable optical paths having a numerical relationship witheach other and with the first time-quantifiable optical path. Thesemiconductor laser is configured to provide a pulsed output at a firstperiodicity and wherein the recombined pulse train at the second nodalpoint provides a pulse train having a second periodicity, the secondperiodicity being a multiple of the first, the multiple defined by thenumerical relationship between the additional time-quantifiable opticalpaths and the first time-quantifiable optical path.

As an example, the semiconductor laser operates at a frequency of about1 gigahertz and the lengths of the time-quantifiable optical pathsdiffer from one another by about 0.2 nanoseconds, so as to define a 5gigahertz pulse train at the second nodal point. Characteristically,time quantification of the optical path length is defined by thedistance required for a pulse to travel at the speed of light for agiven time interval.

In an optoelectronic timing system, an adaptive frequency generatorsystem includes at least one semiconductor laser configured to issuesubnanosecond optical pulses defining a periodic pulse train. Multipleoptical waveguides may be configured to have physical lengths differingfrom one another by a numerical relationship, each length defining atime-quantifiable optical path for a pulse of the train based upon thetime required for a pulse to travel a particular length at the speed oflight. A first nodal point may be coupled to the optical waveguideswhere pulses of the train are directed into the optical waveguides. Asecond nodal point may be coupled to the optical waveguides where pulsesdirected into the optical waveguides are recombined. The periodicity ofpulses recombined at the second nodal point has the same numericalrelationship with the periodicity of the issued pulse train as thenumerical relationship of the optical waveguides.

In a further aspect, the system may further include a pulse detector anda regenerator coupled to the pulse detector and semiconductor laser. Aregeneration waveguide having a length equal to the longest length ofthe waveguides is coupled to receive pulses from the laser. Theregeneration waveguide is not coupled to the first or second nodalpoints. A pulse traveling the regeneration waveguide is directed to thepulse detector and regenerator so as to trigger the laser to issue anext pulse. The physical length of the regeneration waveguide defines afundamental frequency of the system.

The periodicity of pulses recombined at the second nodal pointaccordingly defines a frequency which is a multiple of the fundamentalfrequency of the system, the numerical value of the multiple being equalto the number of optical waveguides. As an example, if the fundamentalfrequency of the system is 1 gigahertz, i.e., a 1-nanosecond period,five (5) waveguides differing from one another by 0.2 nanoseconds woulddefine a five (5) gigahertz pulse train. Lengths of the waveguides aredefined in accordance with a numerical relationship based on thedistance required for an optical pulse to traverse in one (1) nanosecondwhile traveling at the speed of light in the medium defining thewaveguide.

Waveguides may be disposed in semiconductor material, provided asoptical fiber, doped or undoped, or provided as a free-space path ineither a vacuum or a gaseous ambient.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects and advantages of the presentinvention will be more fully understood when considered with respect tothe following specification, appended claims and accompanying drawings,wherein:

FIG. 1 is a simplified, semi-schematic output trace of laser pulseamplitude and pulse width as a function of input pulse amplitude andpower;

FIG. 2 is a simplified, semi-schematic structural diagram of oneembodiment of a laser retriggering circuit in accordance with theinvention;

FIG. 3 is a simplified, semi-schematic structural diagram of a secondembodiment of a laser retriggering circuit in accordance with theinvention;

FIG. 4 is a simplified, semi-schematic diagram of one embodiment of anoptical timing system in accordance with the invention;

FIG. 5 is a simplified, semi-schematic diagram of a second embodiment ofan optical timing system in accordance with the invention;

FIG. 6 is a simplified, semi-schematic diagram of an additionalembodiment of an optical timing system in accordance with the invention,including a tiered configuration;

FIG. 7 is a simplified, semi-schematic diagram of one embodiment of aprecision optical timing system in accordance with the invention;

FIG. 8A is a simplified, semi-schematic diagram of one embodiment of apulse delay optical compensator circuit in accordance with theinvention;

FIG. 8B is a simplified, semi-schematic diagram of one embodiment of apulse advance optical compensator circuit in accordance with theinvention;

FIG. 8C is a simplified, semi-schematic diagram of one embodiment of acombination pulse delay and pulse advance optical compensator circuit inaccordance with the invention;

FIG. 9 is a simplified, semi-schematic diagram of one embodiment of anadaptive pulse phase compensator circuit in accordance with theinvention;

FIG. 10 is a simplified, semi-schematic circuit diagram of oneembodiment of a logical gate implementation for pulse delay and advancein an optical compensator circuit in accordance with the embodiment ofFIG. 9.

DETAILED DESCRIPTION

Continuous wave laser diodes are well known in the semiconductor laserarts. They provide a low cost and physically small solution fordevelopment of optical and optoelectronic systems. While very useful inoptical data transmission applications, continuous wave lasers havesignificant disadvantages when used in optical timing applications, notthe least of which is their conventional inability to deliver a pulsedoutput signal with a sufficiently high output power. This is quitedisadvantageous when a continuous wave laser is coupled to an opticaltransmission path such as a waveguide or optical fiber.

Optical transmission paths are known to attenuate laser signal energy aswell as disperse the output waveform (a process termed pulse spreading)thereby requiring periodic electronic amplification and pulse squaringcircuitry to be provided in the signal path. Where the initial outputsignal is relatively weak, such amplification and shape processing mustbe provided more frequently, resulting in expensive and highly complexinstallations. Accordingly, although able to be operated in a pulsemode, continuous wave semiconductor lasers have not been considered fortiming applications because of what is conventionally considered theirinherently limited output power characteristics.

It has been determined, however, that virtually all continuous wavelaser diodes are able to be operated in a certain manner in order toachieve a high power pulsed output by operationally exercising themusing a sub nanosecond input pulse having an IV (power) characteristicat or exceeding a particular derived current-voltage (IV) thresholddescribed in more detail below. The operation of continuous wave laserdiodes in this manner is not described in any manufacturer's data sheetsnor are they currently known by those having skill in the art. Indeed,the particular current-voltage thresholds required to operate theselasers in a high output power pulse mode is far above the manufacturer'soptimum operating levels, and indeed far exceeds the nominal inputthreshold for any semiconductor laser diode examined.

Although the input characteristics are far in excess of nominaltolerance levels, this is not considered to pose any operationaldangers, because the input pulse is defined as having a duration of lessthan about one nanosecond and a duty cycle of about less than 25%.Consequently, there is not enough time for heat accumulation to occurand therefore no thermal damage is imparted to the laser material. Theuse of continuous wave lasers in this manner is not isolated to anyparticular laser diode composition nor is it limited to lasers operatingwithin any particular wavelength regime. The operational characteristicsof the present invention have been demonstrated on various lasercompositions including AlGaIn, AlGaInP, GaAlAs, AlGaAs, and the like,having wavelengths of from about 600 nanometers to about 1600nanometers.

Even for those few continuous wave laser diodes that do have a pulsemode defined by the manufacturer, the stated output for these diodes istypically far below what one is able to achieve for a pulsed output inaccord with the invention. For example, the Hitachi HL7581G can bepulsed per the manufacturer's specifications at approximately 2V but isonly capable of achieving an output pulse with a power characteristic offrom about 50 to about 60 milliwatts. A continuous wave laser operatedin accordance with the present invention is able to define a pulsedoutput exhibiting a considerably higher output power; in the case of theHitachi HL7581G, operating the laser diode in accordance with theinvention will allow one to achieve a 3000 milliwatt output, i.e., abouttwo orders of magnitude higher output power than previouslycontemplated.

Specifically, it should be understood that the majority of thesemiconductor continuous wave lasers are designed to operate with aninput voltage of from about 2.5 V to about 3 V and require an inputcurrent of from about 50 to about 150 milliamps, for continuous waveoperation. In the case of the Hitachi HL7581G, mentioned previously, thespecified input threshold current is about 45 milliamps (at 25° C.) andthe output power, as a function of injection current, is specified tofollow an approximately 0.5 milliwatt per milliamp slope (conventionallytermed a 0.5 mW/mA slope efficiency). Nominal operating current is about140 milliamps at the nominal 50 mW output.

Where a pulse mode is explicitly stated, the operational characteristicsare such that they remain generally within the specified input andoutput constraints of continuous wave operation. In the Hitachi HL7581Gcase, pulsed operation is allowed with a pulsed optical output power ofabout 60mW, a 50% maximum duty cycle and a maximum pulse width of about1 microsecond. Typically, the maximum continuous wave output ofconventionally operated continuous wave devices ranges from about 5 toabout 70 milliwatts, for typical laser diodes having nominal inputimpedances of generally less than 1 Ohm (typically in the range of about0.2 Ohms). When the maximum specified injection current (I) isconsidered for a range of continuous wave laser diodes, along with theirspecified maximum operating voltage (V), is it relatively simple toderive a range of corresponding operational input power characteristics(IV=W) for these lasers of from about 0.025 watts to about 0.35 watts.

In accordance with the present invention, a continuous wave laser may beoperated at a particular input regime, characterized by a particularoperational input power characteristic (a current-voltage thresholdtermed herein the Siepmann Threshold or ST) in order to obtain a highoutput power pulse in the subnanosecond range. This threshold (ST) isexperimentally determined for each laser diode, and typically lies inthe range of from about 2.0 to about 6.2 Watts, depending on theparticular laser diode composition and construction under investigation.Once the ST threshold has been determined for each laser diode, thatlaser may be operated to obtain output pulses having similar powercharacteristics, i.e., output pulses in the range of about 3000milliwatts. Although not particularly relevant to practice of theinvention, it has been observed that the Siepmann Threshold appears tohave a direct proportionality relationship to the surface area of thelaser diode at issue, all other factors (such as composition andwavelength) being equal.

In particular, the lowest current-voltage threshold necessary to achievea subnanosecond high power pulsed output from each of the continuouswave laser diodes evaluated, ranged on the order of from about 12 toabout 160 times the manufacturer's maximum input power ratings for theparticular laser diode at issue. The Siepmann threshold may be obtainedfor virtually any continuous wave laser diode, with the maximum outputof each diode being found somewhere in the range of from about one toabout two times the ST threshold for that diode.

Operationally, and in accordance with principles of the invention, theSiepmann Threshold (ST) is found by applying a substantially increasedinjection current to the device at an increasing operating voltage (Vop)and evaluating the device's output characteristic. The current injectionis provided in pulse fashion and is generally in the range of from about200 picoseconds to about 600 picoseconds, but it could be substantiallyless. The pulse width should, however, be maintained in the range ofless than 1 nanosecond. In order to avoid damage to the diode, it willbe necessary to maintain the input pulse at a duty cycle of less thanabout 25%, and preferably less than about 20%. Input current andoperating voltage are increased until the device's output characteristicexhibits a substantial and quite surprising jump in measured outputpower. Notably, the output power increase is not linear. Output powerremains generally within specified tolerances until the ST threshold isreached for each diode. At the ST threshold, however, the output powercharacteristic jumps at least one order of magnitude and typically twoorders of magnitude.

It should further be mentioned that the ST threshold may be convenientlyfound by starting the procedure utilizing the laser diode manufacturer'srated maximum operating voltage and injecting an operating current inthe range of about 1 Amp. From this starting point, one having skill inthe art can easily determine a set of current-voltage matrix values thatwill identify the point at which the ST threshold determines operationof the device. Current may be swept, with voltage incrementally steppedfor each sweep, or vice versa. Alternatively, a set of IV “corner”values may be generated and IV sweeping performed about the corners inaccord with well understood principles of experimental statistical dataanalysis.

The minimum target input power is about 2.0 watts, with several devicesexhibiting ST thresholds in the range of about 6 watts. Notably, itwould appear that increases in the operating voltage have a morebeneficial effect in deriving the ST threshold than increases in theinjected current. Pulsing a CW laser diode with an input pulse amplitudein the 4000 to 5000 millivolt region, while maintaining the inputcurrent in the 1 Amp region seems to be able to develop the ST thresholdfor most devices.

Turning now to FIG. 1, there is shown an exemplary output characteristiccurve developed for a typical continuous wave laser diode operated atand above its derived ST threshold. In the example of FIG. 1, a highoutput subnanosecond pulse is developed at an observed ST thresholdcorresponding to approximately 4.0 watts. Data was taken utilizing aninput current of relatively constant value at approximately 1.0 Amps;thus the input pulse amplitude is characterized in terms of voltage(i.e., an ST threshold of about 4000 mV). When so operated, a photonicpulse develops at the threshold and exhibits an initial output pulseamplitude characteristic in the range of about 200 mV, at a pulse widthsubstantially equal to the input pulse width of about 150 picoseconds.The photonic pulse amplitude increases as the input pulse amplitudeincreases until a maximal photonic pulse amplitude (MPPA) is developed.

In the example of FIG. 1, the MPPA is observed to be in the region ofabout 500 mV, or about at least two times the pulse amplitude at the STthreshold. After the maximal photonic pulse amplitude is reached,further increases in the input pulse amplitude will actually cause adecrease in the photonic pulse amplitude until a nadir is reached.Typically, this occurs at about an input pulse amplitude of 5% to 7%beyond the ST threshold amplitude (about 4270 mV in the example of FIG.1). Although a nadir in output pulse amplitude is experimentallyobserved, the output pulse amplitude value at the nadir is stillgenerally in the region of the output pulse amplitude defined at the STthreshold. It has been experimentally determined that the MPPA will notagain be reached after the nadir no matter how much the input pulseamplitude is increased; indeed, the output pulse amplitude is observedto plateau at a level somewhat below (approximately 5% to 10% below) themaximum output pulse amplitude developed at the MPPA.

The output pulse width, which remains generally stable until the postMPPA nadir, is observed to increase as the input pulse amplitudeincreases beyond the value defining the photonic pulse nadir. Outputpulse widths are stretched from the nominal input pulse widths to abouttwice the nominal input value. The example of FIG. 1 indicates outputpulse stretching from a nominal value of about 200 picoseconds to avalue of about 400 picoseconds at an input amplitude of about 15% toabout 20% in excess of the ST threshold. Notionally, the output pulsewidth remains stable at about the input pulse width across the range ofinput amplitudes from the ST threshold to at least the nadir.

Accordingly, it will be understood that operating a continuous wavelaser diode, in accordance with the invention, is able to define adevice which is capable of developing a subnanosecond pulsed output withan output power characteristic significantly larger than conventionallyoperated diodes. Those having skill in the art will understand that arelatively simple continuous wave semiconductor laser diode may now beutilized as an optical timing device due to its previously unknown highoutput power pulse characteristics. The use of continuous wave laserdiodes for subnanosecond pulsing in this manner in order to achieveextremely high outputs is not commonly known to those practiced in theart and indeed represents a highly surprising result. This novel useallows one to achieve a high pulse output with inexpensive laser diodesthat could otherwise only have been achieved with expensive pulse lasersystems costing thousands of dollars. Additionally, this allows smallsemiconductor lasers to be used in combination with semiconductorelectronic circuitry in order to manufacture small and inexpensive highspeed optoelectronic timing devices.

Turning now to the exemplary embodiment of FIG. 2, the novel high powerpulse mode laser diode described above can be adapted to provide alight-based timing/clocking device with operational characteristics thathave not been realized beforehand. In the particular embodiment of FIG.2, the invention is directed to a semiconductor laser which is able todeliver a subnanosecond output pulse having an amplitude characteristicin the range of more than 100 milliwatts at a pulse duty cycle ofanywhere from about 1% to about 25%. The semiconductor laser, indicatedat 10, is overdriven at or above the ST threshold, as described above,and is triggered by an injection current derived from a photoconductiveor capacitive trigger device 12, thereby yielding a high power laseroutput pulse that is less than one nanosecond in duration. In theillustrated embodiment, the semiconductor laser is coupled in serieswith an optical fiber or an optical waveguide 14 which functions as thetiming path element, in a manner to be described in greater detailbelow. When the device is used in a closed-loop mode, i.e., an initialpulse, or a portion thereof, is used to trigger a next or subsequentpulse, approximately 20 percent of the laser initial output pulse is fedback to the trigger (photoconductive or capacitive trigger) 14 where itcauses an additional laser pulse to be generated and propagated into thewaveguide.

As was described previously, the wavelength of the laser pulse, whetherinitial or subsequent, is not as crucial to timing operations as thestable functioning of the semiconductor laser itself. Accordingly, careshould be taken to define the trigger output characteristics such thatthey meet the laser's determined ST threshold, and preferably thelaser's determined MPPA. Additionally it is contemplated that any outputpulse derived from the laser diode is collimated with a collimating lens16, in order to minimize dispersion of the output pulse and maximizeoptical power in each pulse propagated through the fiber or waveguide.

As shown in the exemplary embodiment of FIG. 3, thephotoconductive/capacitive layer may be further defined as including acapacitor-type semiconductor injection trigger 18 coupled to asemiconductor photodetector 20. When the device is used in a closed-loopmode, approximately 20 percent of the laser output pulse is fed back tothe semiconductor photodetector 20 which then sets off thecapacitor-type semiconductor injection trigger 18, in turn retriggeringthe laser 10 to develop a pulse. As was the case in the exemplaryembodiment of FIG. 2, the laser is coupled to an optical fiber orwaveguide which propagates the pulse and includes means for dividingapproximately 20% of the pulse and directing what might be termed thetrigger pulse back to the photodetector for subsequent retriggering.

Particularly suitable types of devices that might be used in thisconnection include MOS photocapacitors similar to those implemented inconventional CCD image capture and reproduction technology. It hasbecome apparent that CCD technology is able to be used in many differentpotential applications, including signal processing and imaging,particularly because of silicon's light sensitivity. Silicon responds tophotons in the optical spectrum at wavelengths less than about 1.0 mm.This is relatively important since the visible spectrum falls between0.4 mm and 0.7 mm and the majority of semiconductor CW laser diodesexhibit dominant output modes in the visible range. The CCD's superbability to detect light has turned it into the industry-standard imagesensor technology.

Further, CCD devices inherently involve a charge transfer mechanism,which is easily coupled as a trigger to a high-speed chargeamplification device such as a voltage follower configured BJT or an SCRcoupled to discharge a capacitor circuit upon receipt of a nominaltrigger current value. Although the illustrated embodiments of FIGS. 2and 3 have discussed photodetectors and capacitor-type trigger devices,it will be understood that these have been discussed only for purposesof example and not to limit the construction and operation of thedevice. Photodetectors and triggers might also be thought of asincluding initiator devices and/or regenerator devices that function tocause the laser diode to issue an appropriate pulse. Accordingly, asuitable regeneration amplifier is implemented as any form of electronicdevice operationally responsive to an optical pulse in the generallyvisible range of the spectrum, and outputting a substantially high powertrigger pulse in the subnanosecond region; such a regeneration amplifiershould be able to switch current throughout at least 500 MHz. Asmentioned above, a bipolar switch might be particularly suitable when itis understood that a high speed timing device should be able to switchthe laser operationally at speeds of about 500 MHz at current levels atabout 500 milliamps.

It should also be understood that a timing device constructed inaccordance with the invention might operate in two different modes, anopen-loop mode and a closed-loop mode. When operating in an open-loopmode, the device might be controlled by a voltage controlled oscillator(VCO) with clock speeds in excess of one gigahertz (1 GHz) beingachieved by a suitable definition of the optical path, as will bedescribed in greater detail below. In a closed-loop mode, as wasdescribed above, the trigger input for subsequent pulse development istaken from at least a portion of the initial, or some previous, pulsefrom a well defined distance along the optical path itself.

In accordance with principles of the invention, an optical timing deviceutilizes the speed of light in an ambient medium traversing a definedtravel distance “d” in order to derive a stable time interval “t”. Anoptoelectronic device, in accordance with the invention, is predicatedupon utilization of the speed of light, with a light pulse traversingeither an optical fiber or a waveguide for a known distance, in order todefine a known time. In effect, timing becomes a function of knowndistance, as opposed to a function of the vagaries of electrical orelectronic switching devices.

For purposes of example, the speed of light is taken to be 3×10⁸ metersper second. Although this is not precise, and also refers to the speedof light in a vacuum, the value of 3×10⁸ meters per second is sufficientfor exemplary purposes. Additionally, the speed of light in other mediawill be understood by those having skill in the art to depend upon theindex of refraction of that medium. While these differences exist, theywill be deemed to be not particularly relevant for purposes ofdescription. The actual values used in the context of the discussionmust be recognized as exemplary only and chosen solely for ease ofcomputation. Accordingly, assuming a light pulse is generated andpropagated down an optical fiber or waveguide, the propagation speed ofsuch a pulse will be on the order of 30 centimeters per nanosecond oftravel time (more particularly about 20 centimeters per nanosecond wherethe index of refraction of a typical optical fiber is on the order ofabout 1.5). Thus, a gigahertz timing signal might well be understood asincluding multiple 30 centimeter paths, with each 30 centimeter pathlength defining the fundamental one nanosecond timing interval.

Turning now to FIG. 4, an optoelectronic timing device, in accordancewith the invention is depicted in a simplified, semi-schematic form andis configured to define a native one gigahertz timing device operatingin an open-loop mode with a 200 megahertz trigger. In the exemplaryembodiment of FIG. 4, a 200 megahertz phase lock loop (PLL) 30 operatesat 200 megahertz and provides a trigger signal to a semiconductor laser32 so as to develop a high power output pulse, in a manner describedabove. The laser 32 develops an output pulse which is propagated over anoptical fiber or waveguide 34 which is substantially 150 centimeters inlength and which further includes taps 36 at equal 30 centimeterintervals along the length of the fiber or waveguide. Each of the taps36 defines a path to a corresponding photodetector circuit or acentrally disposed photodetector circuit 38, so as to define a set ofsequential signal sources, each source defining a timing signal onenanosecond following the previous signal source.

Since the PLL 30 is operating at 200 megahertz, the semiconductor laser32 fires off a new optoelectronic pulse every 200 megahertz. Each pulseis propagated down the fiber or waveguide 34 and sequentially causes thephoto detector 38 to receive a quantum of the optoelectronic pulse andgenerate an electrical signal in operational response thereto. Thus, a200 megahertz PLL is able to define a one gigahertz native timingsignal, without reliance upon integrated circuit electronic switchingcharacteristics. Further, it will be understood by those having skill inthe art, that since the timing properties of the resultant signal dependsolely upon a known distance parameter between photo detectors, theresultant timing signal will be stable and internally self consistentand repeatable with respect to pulses generated by the optical system.

In the exemplary embodiment of FIG. 4, it should be understood that theonly timing jitter introduced to the system will result from timingirregularities of the 200 megahertz PLL. Since it is relatively straightforward to accommodate for jitter and other timing abnormalities in 200megahertz systems, an optoelectronic timing device, in accordance withthe invention, will be understood to exhibit the stability and precisioncharacteristics associated with relatively low speed technologies, whiledefining native timing pulses operating in the gigahertz regime.

It should further be understood that the exemplary embodiment in FIG. 4has been described in terms of an open loop system with optical pulsesinitiated by an external trigger device, such as a PLL. Alternatively,the apparatus can be constructed to operate in closed loop fashion byhaving the vestigial optical pulse retrigger the semiconductor laser 32at the same time that it generates an electrical pulse through the finallength of the fiber or waveguide and thence to the photodetector. Inorder to accommodate this function, the fiber or waveguide could bedisposed in a circular arrangement or by defining a folded optical path,as is well known as those having skill in the art. Accordingly, thesystem should be understood as contemplating open loop as well as closedloop operation without violating the spirit of the invention.

The exemplary embodiment of FIG. 4 was described in order to develop anappreciation of the fundamental features of an optoelectronic devicethat operates in accordance with the invention. The system need not belimited to a 200 megahertz trigger, nor need the system be limited toone gigahertz operation, by defining the individual waveguide segmentsin terms of 30 centimeters. Indeed, a 10 gigahertz optoelectronic timingdevice is able to be produced with current technology, by implementingthe various elements in Gallium Arsenide (GaAs) integrated circuittechnology rather than the more prosaic and conventional silicon. GaAsintegrated circuit chips allow for inherently faster operationalresponse times because of their inherently lower parasitic capacitances,as well as allow a semiconductor laser to be fabricated in situ duringthe device manufacturing process. Such a laser system (termed a verticalcavity surface emitting laser or VCSEL) would negate the need forproviding and bonding a separate semiconductor laser chip onto a siliconintegrated circuit. The cost savings more than compensate for theadditional manufacturing costs of GaAs integrated circuit technology.

In order to overcome the output power limitations known to apply toVCSEL lasers, multiple VCSELs are fabricated in GaAs and directed into a100 micron lightguide. 10 VCSELs may easily be fabricated in GaAs to fitinto the footprint required for a 100 micron lightguide. Thus, 10 VCSELscan be triggered simultaneously into the waveguide in order to obviatethe output power limitations of an individual laser.

From the discussion of the exemplary embodiments of FIG. 4, it will bestraight forward for one having ordinary skill in the art to understandthat a 10 gigahertz optoelectronic timing device will have a 2nanosecond loop time for a single laser (or simultaneously initiated setof lasers). A 2 nanosecond loop time is accomplished by utilizingappropriately devised path lengths hosting optical pulses generated by aclosed loop system, as described above, or by utilizing a 500 megahertzoscillator (PLL) to trigger the laser system. In this regard, theoptical pulse width is contemplated as being less than 200 picosecondsand, preferably, less than 100 picoseconds when the system is utilizedas a 10 gigahertz clock.

It should also be noted, that for a 2 nanosecond loop time, the overalllength of the optical fiber or waveguide which is required to support 2nanosecond travel time will be on the order of 60 centimeters.

Turing now to the exemplary embodiment of FIG. 5, there is shown insimplified, semi-schematic form, a 10 gigahertz timing system, generallysimilar to the exemplary embodiment of FIG. 4, but including a cascadelaser system defining two timing portions. Each of the portions isgenerally similar to the embodiment of FIG. 4, with a semiconductorlaser 32 being triggered by a PLL trigger device, or equivalent, and isshown as directing optical pulses down a primary optical fiber orwaveguide 33, including suitably disposed taps leading to a primaryphoto detector 34. However, as each pulse generates a signal in theprimary photodetector, the primary signal is seen as triggering asecondary laser 40, which, in turn, develops an optical pulse down thesecond portion of the system.

The second portion of the timing system in FIG. 5 is structurallyidentical to the first portion and only differs from the first portionin having its laser triggered by each vestigial optical pulse as opposedto being triggered by an electronic timing oscillator, for example. In asense, the exemplary embodiment of FIG. 5 can be thought of as a quasiopen and closed loop system in combination. It will further beunderstood that the system may be implemented in a completely closedloop fashion by merely completing the circuit between the second portionof the timing device and the pulse initiation laser 32 in a mannerdescribed above in connection with FIG. 4. Briefly the exemplaryembodiment of FIG. 5 is provided solely as an example of how lasers andfiber/waveguide may be cascaded in order to define primary and secondarytiming loops. In this regard, it should be understood that a 10gigahertz timing device may be formed utilizing several possiblecombinations of trigger circuits, lasers, waveguides and taps.

For example, a 10 gigahertz timing device might include a single opticalloop, utilizing a single initiation laser system and including 10outputs, each cycling through 1 nanosecond. Where the system is closedloop or fired by a 1 gigahertz clock trigger, a simple constructionanalysis reveals that the system operates at 10 gigahertz. Additionally,such a system might be implemented as a single unit including 20 outputseach cycling at 2 nanoseconds where the laser is fired in accordancewith a 500 megahertz clock trigger device.

In a two-laser system, such as depicted in the exemplary embodiment ofFIG. 6, a secondary laser 50 is nested within the primary timing loop,such that a trigger pulse developed by the primary loops' photo detector52 initiates an optical pulse in the secondary laser system 50. Theprimary loop cycles at 100 megahertz, by being triggered by a 100megahertz PLL 54, for example. The primary loop further includes tenequally-spaced outputs each having a length defining a 10 nanosecondinterval. As a pulse traverses a particular leg of the loop, itgenerates a signal in the primary loops' photo detector, 52 which, inturn, fires off the secondary laser 50. The secondary laser 50 is,therefore, triggered at a 1 gigahertz rate and cycles at 1 nanosecondintervals, by defining a secondary waveguide 56, itself having tenequally-spaced apart outputs so as to define a 10 gigahertz pulse trainto the secondary photo detector 58.

Similarly, a 10 gigahertz system might be implemented as primary andsecondary units with the primary cycling at 100 megahertz and includingfive outputs: the secondary cycling at 2 nanosecond intervals, i.e.,twenty outputs, thereby defining a 10 gigahertz pulse train. Thisconceptual construction may be easily extended to a three-laser systemby an additional level of nesting. A tertiary laser system need only becoupled into the secondary system, such that the tertiary laser is firedby the secondary photo detector electronics, and so forth. Since theactual timing rate is nothing more than an appreciation of a physicaldistance down a waveguide of optical fiber, it will be immediatelyunderstood by those having skill in the art that any number ofmechanical arrangements may be implemented so as to define opticaltiming pulses having any number of desired characteristics at virtuallyany technologically feasible speed. Indeed, one of the particularutilities of the invention is its ability to generate gigahertz regiontiming signals utilizing very simple and straightforward, relativelylow-speed electronic trigger pulses. A 100 megahertz oscillator or PLLis all that is required to initially fire off the system into gigahertzand multiple gigahertz pulse train definition.

Now that the fundamental constructor in operations of an optical timingsystem, in accordance with the invention, has been described, it wouldbe worthwhile to discuss a few of the timing systems that can beimplemented utilizing such technology. Turning now to the exemplaryembodiment of FIG. 7, there is shown a precision serial optical timingdevice, termed herein a “lightclock”, that utilizes various aspects ofthe novel optical timing system, described above. In the exemplaryembodiment of FIG. 7, the precision serial lightclock is implemented asa set of nested timing loops, with each timing loop having a loop timeof substantially one order of magnitude smaller than the loop time ofthe proceeding. Specifically, the exemplary embodiment of FIG. 7describes six optical timing loops, nested within a main, or seventh,loop defined by implementing the system in a closed-loop fashion. Eachof the optical loops is defined by an optical fiber which, since therefractive index of optical fiber is typically 1.5, defines anapproximately 20 centimeter distance for each required nanosecond.

As described previously, a semiconductor laser 60 is fired by an initialtrigger pulse to define an optical pulse into a first timing loop 62.The first timing loop 62 is contemplated as defining a 0.01 millisecondloop, thereby requiring approximately a 20 kilometer length of fiber,with an approximately 2 kilometer spacing between each of the 10 outputsof the loop. The first optical timing loop 62 defines an output byhaving each of the fiber taps provide a signal to a centrally disposedphoto detector 64 which, in turn, provides the trigger signal to anext-level laser diode 66 which develops an optical pulse to thenext-level optical loop 68. The next-level optical loop 68 is defined asa 1.0 microsecond loop, thereby requiring an overall 2 kilometerdistance for the loop fiber, with each of 10 outputs being spacedapproximately 200 meters apart. As before, each of the outputs isdirected to a centrally disposed photo detector 70 which necessarilyreceives optical signals at one microsecond intervals. Further, thephoto detector 70 provides the necessary trigger pulse to a next-levelsemiconductor laser 72 which defines an optical pulse into thethird-level (0.1 microsecond level) optical path 74. Spacing betweenoutputs in the third-level optical path are necessarily 20 meters apartin order to define a 0.1 microsecond pulse train to the centrallydisposed photo detector 76.

Similarly, fourth, fifth, and sixth optical loops (78, 80 and 82,respectively) function just as described previously, with theirrespective output spacing being 2 meters, 20 centimeters and 2centimeters, in order to ultimately provide a 100 picosecond pulse trainfrom the final timing loop.

Returning momentarily to the first-level loop of the system, it shouldbe noted that the 20 kilometer fiber terminates in a further photodetector 84 which is used to retrigger the initial semiconductor laser60 in closed-loop fashion. Accordingly, the initial semiconductor laser60 operates at approximately 0.1 millisecond intervals. Also, each ofthe photo detectors (64, 70, 76, and the like) have an output coupled toa corresponding incremental counter (indicated collectively at 86) thatoffers a count-up methodology by which seconds, minutes, hours, days,etc, can be simply and easily accounted for in order to translate theoptical timing loops of the system into a rigorous and extremelyaccurate precision time keeping device.

The system described in the exemplary embodiment of FIG. 7 can be easilyconstructed by having a semiconductor-type laser pigtailed to an opticalfiber, such as SMF 28, or the laser might be a component of anoptoelectronic integrated circuit (OEIC) or a component of a printedcircuit board. Either a printed circuit board or an OEIC implementationis able to incorporate the optical fibers, photo detectors,initiation/trigger driver, and integral lasers, or any subset of thesecomponents. Additionally, a printed circuit board or OEIC implementationis able to accommodate a free space adjustment gap, between the opticalfibers and photo detectors, at any level, such that fine-tuning can beprecisely performed for path length variations. Such fine-tuning allowsmanufacturing tolerances of the fiber system to be less rigid and isalso able to compensate for any changes in path length that might occurafter component placement or replacement.

Optical pulse division is performed by either a waveguide or withsequential optical taps disposed along the length of the optical fiber.In the exemplary embodiment of FIG. 7, ten optical outputs are definedat each level, but this is only for purposes of convenience ofdescription. Any number of optical outputs might be provided for any andall of the different levels of the optical loops depending upon thedesire of the system designer. Further, optical taps may be configuredto pass any percentage of the input optical pulse that is required forsystem performance. As is well understood by those having skill in theart, optical taps may be defined that tap 10%, 25%, 50%, 75%, or thelike, of the input optical pulse. Likewise, a waveguide division of anoptical pulse may be of any percentage desired by system requirements.By varying the percentage of a pulse that is transmitted into branchingfibers, it is possible to make the output pulse amplitudes all equal, ormake the output pulse amplitudes different for each branch in the loop.Suitably, a laser pulse may be split after the laser by means of an OEICwaveguide, a waveguide on a printed circuit board, a waveguide disposedin line with an optical fiber, or by means of individual fiber tapssplitting off a single fiber. The latter option decreases fiber bulk,but care should be taken to minimize defects in any of the fibers orfiber branches. However, an error in measurement of an individualbranches' length (fiber or waveguide) could be compensated for byadjusting any available free space gap disposed prior to the photodetector. Inline amplification, where necessary, may be providedanywhere within the optical path by a rare-earth doped amplifier (i.e.,EDFA) as is well understood by those having skill in the art. If inlineoptical amplification is required, it would be generally desirable toutilize a 1550 nanometer laser in order to accommodate well understoodamplification methodologies. This is the only foreseeable constraint onlaser wavelength.

In order to compensate for any possible relativistic effects that mightoccur during the usage, an optical comparator, to be described ingreater detail below, can be disposed in any of the loops, but ispreferably disposed in the initial loop of the system. The opticalcompensator is a device for relatively advancing or delaying an opticalpulse within a set pathway by comparing two light pulses and divertingone or the other pulse into a delaying or advancing side pathway, as ameans to get the two light pulses into synchronization.

In an additional aspect, the invention contemplates the use of a sideoptical pathway to advance or delay an optical pulse. A methodology forcomparing two light pulses and diverting one or the other of them viaoptical switches (electrical or optical triggered) into a delaying oradvancing side pathway defines a means to get the two light pulses insync with one another. A methodology for changing the relative position(phase) of a particular light pulse with respect to that of anotherlight pulse by using optical switches to divert said light pulse intodelaying and advancing side pathways is also described.

In an optical timing device, as described above, it is relativelyimportant to compensate for any phase differences developed between aprevious pulse and a newly generated pulse. In this regard, an opticalcompensator (OC) defines a device that can be used within an opticaltiming generator, or any device which needs to advance or delay a lightpulse, or to compensate for any internal or relativistic effects thatmay occur within a device during the transmission of a light pulsewithin a pathway.

Optical compensators are used to adjust the relative phase of a newlight pulse within the transmission pathway so that it is in sync with apreviously generated light pulse or to just advance or delay a lightpulse within the transmission pathway.

The optical compensator device suitably includes a side path or a set ofside pathways where the light pulse is diverted in order to advance ordelay its time characteristic (phase) relative to the main pathway.Because each side path is defined in accordance with a particulardistance metric, it will be understood that an advancing path willnecessarily be shorter than the main pathway. Likewise, a retarding pathwill suitably include a longer distance than the main pathway. Travelalong side pathways is implemented by optical switches which divert thelight pulse into said side pathway when the optical switches are turnedon. Characteristically, optical switches are simply logic gates thatdefine an optical pathway into one branch (waveguide or fiber) uponoccurrence of a particular logical state and define an optical pathwayinto another branch upon occurrence of a second logical state orcondition. Optical switching into such pathways can also be made tode-synchronize pulses or to further advance or delay a particular pulserelative to another by changing the logical gating mechanism being used,as will be well understood in the art of logical integrated circuitdesign.

The individual pathways can be implemented either as a waveguide pathwayor an optical fiber pathway, as described previously. Further, opticalswitches can be of any make that can meet the speed requirementsnecessary for the device. The optical switches can either beelectronically or optically triggered and are contemplated as beingall-optical (optical core), optical-electronic-optical (o-e-oconfigured), optical-mechanical-optical, and particularly implemented assoliton switches. Each of these component types are well known to thosehaving skill in the art and further discussion of their construction andimplementation would not be particularly germane.

A simplified schematic for a delay-type optical compensator, in its mostbasic form, is given in the exemplary embodiment of FIG. 8A, while asimplified schematic for an advance-type optical compensator, in itsmost basic form, is given in the exemplary embodiment of FIG. 8B. Inboth exemplary embodiments, a sidepath 100 is coupled to a main opticalpathway 102 by an optical switch 104. As a pulse traverses the system,and it is determined that its phase must be adjusted, the optical switchdiverts the pulse either onto an advancing path, a delaying path, orallows the pulse to traverse the main pathway, if desired. Phaseadjustment comes into play, for example, if there were a repeatablephase distortion introduced by the pulse regeneration/laser retriggeringcircuitry. Necessarily, this would cause more of a phase stretch,compelling a more often use of a phase advance pathway.

A simplified schematic for an optical compensator, which is able toeither advance or delay an optical pulse such that it is in synchronousrelationship with a previous pulse, in the main pathway, is shown in theexemplary embodiment of FIG. 8C. In this embodiment, two opticalswitches 106 and 108 define entries into delay path 110 and/or advancepath 112, respectively, relative to the main optical pathway 114.

Further, an optical compensator can be configured to adaptively advanceor delay a light pulse such that it is in synchronous relationship witha previous light pulse. One particular embodiment of this feature wouldallow for compensation for any relativistic, electronic phase shift, orintrinsic mechanical changes that may occur such that would make a newlygenerated light pulse be out of sync with a previously generated one. Asimplified schematic of an exemplary embodiment of such an adaptivephase compensator is shown in FIG. 9.

Specifically, optical switches 116 and 118 are controlled by simplelogic gates (electronic or optical) that appropriately turn on thecorrect switch in order to adaptively adjust the phase of a light pulse.The desired action is to turn on optical switch 118 if the new pulse isahead of the previous one; the switch 118 remains in the off state forall other conditions, while the desired action for optical switch 116would be to turn on if the previous pulse is ahead of the new one.Further, operation of each of the switches is adaptive in that theiroperational state is controlled only by the arrival times of both theprevious and the new pulse, and not by any external constraints. Onerelatively simple methodology for accomplishing this feature is to havetrigger edges (rising or falling edges) of each of the light pulses atissue condition a set of simple logic gates, the Boolean arguments ofwhich define the required function. In the exemplary embodiment of FIG.10, a simplified gating diagram for this feature describes an advanceand delay switch implemented using a combination NOT-AND gate setup,with inverted input conditions defining whether the gate is operativelyresponsive as an advance gate or a delay gate.

The rise time for the optical switch is required to be less than thetime it would take the new pulse to arrive via the main pathway. Thefall time for the optical switch would also be required to be fasterthan the loop time of the main pathway and slower than the greatestpossible discrepancy that could occur between the new pulse and theprevious one. Since the advance and delay pathways are of fixed length,multiple sequential optical compensators could be used with each onehaving a shorter pathway until they are in sync within the acceptablemargin of error (which would typically be less than the pulse width ofthe light pulse). This can be relatively easily implemented with aswitch fabric core configured optical switch (a 1×N configuration).

The optical timing device (lightclock), in accord with the invention isalso able to operate as a pulse regenerating closed loop system. Theoptoelectronic timing device is configured as an OEIC including anintegrated circuit chip including either a superimposed waveguide orattached optical fibers as the optical pulse propagation path. Thelightclock is contemplated as having multiple operationalcharacteristics and is constructed with multitiered loops and/or withmultiple lasers: laser(s) for each tier and/or lasers simultaneouslybeing fired for a higher output.

The optical timing device can be used as a simple pulse (optical orelectrical) output device with a frequency in the MHz or GHz range, orhigher as is desired by system timing requirements. The optical timingdevice is further designed to provide an “intelligent” clockingfunctionality with a designed patterned output. The patterned output iscontemplated as including any desired interval between the individualpulses as well as contemplating a predefined variable set of outputamplitude characteristics, such that the periodic occurrence of one of amultiplicity of different amplitude pulses defines an internallyrecursive set of timing triggers. For example, it is well within thecontemplation of the present invention to provide an optical timingdevice that is able to define a 500 MHz timing signal characterized by300 mV pulse amplitudes while the same pulse train defines a 1 GHztiming signal characterized by 200 mV pulse amplitudes. The 200 mVpulses are interspersed within and between the 300 mV pulses to define amodulated pulse train.

Returning momentarily to the exemplary embodiment of FIG. 6, it can beeasily seen that the secondary loop may be devised with a photodetectorand ancillary electronics configured to output 200 mV pulses, while theprimary loop is configured to output 300 mV pulses. Accordingly, thesystem can be adapted to output a combination 1 GHz and 10 GHz signal,with the 10 GHz signal internally dependant on the characteristics ofthe 1 GHz loop.

As described in various other previous exemplary embodiments, theoptical timing device can be implemented as either an open or closedloop device. The particular configuration discussed in connection withthe exemplary embodiment consists of a single loop or a tiered loopsystem with a regenerated pulse between each loop. The output can bephotonic into a waveguide, optical fiber, or collimated into free space.The output can also be electrical via the addition of a photodetector.In order to provide for an optical pulse having a desirable high pulseamplitude, multiple lasers can be ganged and simultaneously fired, asingle laser (or multiple lasers) may be operated at or above its STthreshold, or a single laser (or multiple lasers) may have its outputamplified in conventional fashion. If multiple lasers are usedsimultaneously then each could be provided with its own initialwaveguide pathway or multiple lasers could be merged via fiber splicingor waveguide pathway convergence.

As described in connection with the embodiments of FIGS. 5 and 6, ifeach additional elemental pathway provides an additional travel time ofa known amount (an additional 0.2 ns, for example) and the primary loopdefined a 1 ns cycle then this particular device would exhibit a 5 GHzoutput. A suitable photodetector/pulse regenerator (PD-R) could be assimple as a photoconductive switch with a capacitor or an opticallytriggered FET. An initiator (I) could be as simple as a capacitortrigger to initiate a laser pulse and start the timing cycle.Termination of the closed loop cycling could be achieved by breaking thecircuit anywhere between the PO and the laser.

In the multiple laser case, a ganged laser set-up is simultaneouslyfired, but each laser has its optical pulse directed into a waveguide orfiber having a different length. Thus, a single trigger event causespulse with multiple arrival times by virtue of the different traveldistances. By allowing the different travel paths to be manufacturedwith different attenuation characteristics, each of the arriving pulsesmay be characterized by unique pulse amplitudes and even unique pulsewidths. By varying the size and the alignment of a waveguide pathway orthe splicing percentage of an optical fiber system, differentvoltage/wattages (electrical/optical output respectively) can beobtained for each discrete pulse in the loop cycle if desired. Also bychanging distance between different pulses, the intervals between eachpulse in the loop cycle can be different. Various other internalmodulation schemes can also be accommodated as desired by the systemdesigner using well known optical and/or electronic modulationmethodologies.

Those having skill in the art will immediately understand that severalchanges and modifications may be made to the embodiments disclosed anddescribed without departing from the scope and spirit of the presentinvention. The illustrated embodiments allow for a simple conceptualunderstanding to be had of the present invention and it will beunderstood that the invention is quite adaptable to numerousrearrangements, modifications and alterations. Accordingly the inventionis not to be limited to the specifics of the illustrated and describedexemplary embodiments, but is rather to be defined only with respect tothe full scope of the appended claims.

1-16. (canceled)
 17. An optoelectronic timing system comprising: at least one semiconductor laser configured to output a train of optical pulses at a rate defining a first frequency; a first optical waveguide that is subdivided into a plurality of segments, each of the segments and the first waveguide defining an optical path for the train of pulses; a pulse detector coupled to respective terminal portions of the segments so as to issue a signal upon detection of a pulse traversing each of the segments; and wherein the pulses received at the pulse detector and the signal from the pulse detector have a second frequency that is a multiple of the first frequency, the multiple depending on the number of segments of the first optical waveguide.
 18. The system according to claim 17, further comprising: one or more additional optical waveguides, each additional waveguide being further subdivided into a plurality of segments, each segment having a length equal to the other segments of the additional waveguide, each segment and respective additional waveguide defining a time-quantifiable optical path based upon the time required for a pulse to travel the segment or additional waveguide at the speed of light; one or more additional pulse detectors respectively coupled to the one or more additional waveguides, each additional pulse detector being coupled at a terminal portion of each segment of the additional optical waveguide associated with the additional pulse detector so as to output a signal upon detection of a pulse traversing the segment; and wherein, the first optical waveguide and the additional optical waveguides are disposed in a sequence, such that each pulse detector operationally controls development of a pulse in a next waveguide in the sequence.
 19. The system according to claim 18, further comprising a semiconductor laser disposed to direct an optical pulse into an associated one of the additional waveguides in response to the signal from the pulse detector coupled to the waveguide immediately preceding in the sequence.
 20. The system according to claim 19, wherein an optical pulse travel time along one waveguide of the sequence differs from an optical pulse travel time along an adjacent waveguide in the sequence by one order of magnitude.
 21. The system according to claim 19, wherein each of the additional waveguides has a length that is equal to a length of one of the segments of the waveguide that is just prior in the sequence.
 22. The system according to claim 18, wherein for each of the waveguides, the segments of the waveguide have equal length.
 23. The system of claim 17, further comprising: a first delay waveguide coupled to provide a first optical path for a pulse of the train to the first optical waveguide; a second delay waveguide coupled to provide a second optical path for a pulse of the train to the first optical waveguide, wherein a time required for a pulse to traverse the second optical path differs from a time required for a pulse to traverse the first optical path; and an optical switching system coupled to direct pulses from the semiconductor laser through the first delay waveguide or the second delay waveguide depending on timing of the pulses relative to prior pulses returned from the first optical waveguide. 